Add label sklearn estimator
	dependencies : sklearn, scipy, numpy, skimage
	The documents with labels are used to train a collection of
	supervised classifiers.
	Each label got a classifier (OneVsAll strategy)
	The prediction are made at index time : non labelled documents
	are filled with an estimated predict_label_list
	The whoosh search takes into account the predict_label_list
	Older documents may not be correctly predicted according to the
	new document (online learning), so it can be usefull to reindex all
	TODO: we might want to save the thumbnail in a cache file for each doc
Add prediction to doc member and gui (bold unchecked labels)